Encyclopedia Astronautica
1964.04.16 - Apollo changes for Block II


Joseph F. Shea, ASPO Manager, in a letter to North American's Apollo Program Manager, summarized MSC's review of the weight status of the Block I and the design changes projected for Block II CSM's.

The Block II design arose from the need to add docking and crew transfer capability to the CM. Reduction of the CM control weight (from 9,500 to 9,100 kilograms (21,000 to 20,000 pounds)) and deficiencies in several major subsystems added to the scope of the redesign.

Redesign of the CM would cause a number of changes above the deck, although ASPO believed that the 73.7-centimeter (29-inch)-diameter tunnel could be retained and tunnel access might be improved if the restrictions for seating the hatches were removed. Other changes not related to the docking and transfer requirement would be considered as long as they did not affect the structure below the deck.

Changes below the deck would be kept to a minimum on both the inner and the outer structure. Anything which might invalidate the applicability of the Block I lunar reentry tests to the Block II design would not be changed.

ASPO wanted to evaluate a preliminary design of the CM in which the only access to the LEM would be by extravehicular transfer. Although this approach was not currently considered operationally acceptable, any gains from such a design should be studied.

ASPO agreed that the CM thermal protection would be enhanced by addition of a boost protective cover for both Block I and Block II. A "soft" cover should be simple to design and operate, and a boost cover would permit coating the CM with a thermally efficient surface. This, with the help of attitude programming, should permit North American to reduce the initial ablator bond line temperature from 394 K (250 degrees F) to below 338 K (150 degrees F). ASPO also asked the contractor to consider raising the bond line temperature on the blunt face from 590 K (600 degrees F to 700 K (800 degrees F). These changes would reduce ablator weight significantly.

To eliminate the humidity problem in the Block I subsystems, ASPO believed that electronic repackaging would be required. Such a redesign should take advantage of ASPO's decision to eliminate onboard maintenance as an acceptable means of achieving mission reliability. A more efficient mounting arrangement should be considered in conjunction with electronic system repackaging. Elimination of onboard maintenance would change requirements on the inflight test system; perhaps that system could be eliminated from the spacecraft.

The biggest uncertainty in weight requirements was meteoroid protection. The design approach to this problem should be incorporated with a redesign of the SM to reduce both the tank size and structure (but see August 6 statement of Robert O. Piland) consistent with a 16,800-kilogram (39,000-pound) consumable fuel load, rather than the current 20,400-kilogram (45,000-pound) capacity, The SM design concept should remain the same, but North American should use this opportunity to clean up several structural details.

The SM thermal control system should be passive. Spacecraft orientation, either on a semicontinuous or discrete attitude program, would be permissible to maintain necessary temperature limits. To reach acceptable thermal time constants, the reaction control system (RCS) might have to be modified. It might also be desirable to change the RCS fuel to monomethylhydrazine.

Because of the large amount of spacecraft wiring, North American was asked to study using smaller sizes and reduced insulation thicknesses.

Another consideration was reducing the lunar mission time from 14 days to the reference mission length of about 10 days. But the current tank sizes should be maintained and the spacecraft should be capable of 14- day earth orbital missions with three men. The velocity reserve in the RCS might be decreased if the attitude requirements for guidance and navigation were eased. Here, also, the current tank sizes should not be changed.

Other major changes (such as redesign of the fuel cell, incorporation of new heatshield material, cryogenic helium pressures, and adapter staging) could be considered in the redesign; they would, however, be approved only if the foregoing changes did not provide sufficient weight margin.

ASPO would require a complete preliminary design and impact assessment of the Block II spacecraft before its incorporation into the program would be authorized.

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